During the production of ultra-fine particles by milling, the surface of the milled solid increases approximately reciprocally to the square of the particle size. At the same time, the particle mass decreases by the power of three of the particle size. Because of these physical factors, surface-active forces such as the van der Waal force or electrostatic forces gain a disproportionate effect with decreasing particles size.
This can especially be observed in the range of d50<2 μm, with a heavily increasing tendency when the particles are even smaller. As a consequence, the generated ultra-fine particles will re-agglomerate. The air classifier integrated into the fluidized bed as well as in the high-density bed jet mills to limit the upper particle size prevents the discharge of these agglomerates consisting of the finest particles (which it “detects” as coarse particles) from the mill, so that these are supplied for reprocessing. Milling energy is therefore used again to deagglomerate the already fine particles again, which immediately form new agglomerates again. This results in a large increase in the energy requirement of the milling.